Thermoplastic prepreg is used to make structural parts for various devices strong, rigid, and lightweight. Thermoplastic prepreg is the material resulting from impregnating fiber reinforcements with a formulated resin. These advanced composite materials offer many advantages over conventional steel and aluminum since composite parts fabricated from thermoplastic prepreg materials are generally stronger and stiffer than metals. Components fabricated from thermoplastic prepreg materials also provide greater resistance to fatigue, creep, wear and corrosion than metals.
In use, several thermoplastic prepreg plies with different fiber orientations are assembled into layers and multiple layers are stacked on top of each other to form a layup. The layup is then cut into thermoplastic prepreg segments. The thermoplastic prepreg segments are then assembled into a kit or wedge to form a particular structural part of the device under construction. Assembly of the kits or wedges requires stacking the thermoplastic prepreg segments in sequence and orienting the pieces according to a geometry envelope. By cutting the thermoplastic prepreg segments from layups, the structural parts of the device receive strength in more than one direction. Composite parts made from thermoplastic prepreg have very high strength in the direction of the fibers and very poor strength in other directions.
Layups are useful because they reduce the amount of time required to tailor the architecture and to catalog the thermoplastic prepreg segments of a particular device part. However, since thermoplastic prepreg material has a very low coefficient of friction, or no tackiness, the plies tend to slide, making stacking, cutting, and assembly extremely difficult. The thermoplastic prepreg plies forming a layup must be retained in alignment during and after cutting. If the thermoplastic prepreg plies are not held together during all stages of assembly, it becomes difficult and labor intensive.
After cutting thermoplastic prepreg segments from a layup, it is advantageous to retain the thermoplastic prepreg segments together in alignment during assembly of the pieces into a kit. Some thermoplastic prepreg segments are bulky and some are very small, so working with thermoplastic prepreg segments that are properly held together greatly reduces assembly time and difficulty. Each thermoplastic prepreg segment must be fastened to corresponding pieces as they are stacked and oriented as part of a kit or wedge. Several kits or wedges are typically molded together to form a composite part, such as a cylinder for a sabot.
Thermoplastic and thermoset carbon reinforced prepreg materials both require intermediate processing to reduce the bulk factor, or a method of reducing the thickness of the assembled materials. This process is normally required for compression molding of thick laminates or in this instance a thick cylinder section. To debulk an assembly pressure and/or temperature need to be applied. Thermoplastic materials require a higher temperature than do thermoset materials, which can be debulked with pressure alone because of tack of the resin matrix. The purpose for thickness reduction is to reduce the amount of movement of the individual layers of prepreg material. The orientations of the carbon fibers in each layer are critical to the structural strength of the finished part or assembly of prepreg layers being molded into a preform ready for machining.
Known methods for debulking exhibit some unfortunate drawbacks. For example, ultra sonic welding of kit assemblies lacks sufficient heat control and required pressure and provides uneven heat energy. Another process, ultra sonic tack welding, requires painstaking hand welding of kit assemblies.
In yet another known method, ten-degree wedge compression molding, kit assemblies have been fabricated successfully. Unfortunately, the compression method requires costly tooling, control systems, and a large press. The compression method also suffers from long cycle times of up to four hours and high labor costs. In this method, the tool temperature is raised to melt the thermoplastic resin. The part is compressed. After compression, the part must be cooled down to room temperature before part removal and cleaning and preparation of the tool with mold releases prior to the next cycle. In addition, the molded parts must be sand blasted before a subsequent molding operation can be performed.
Another area where known apparatus and methods are inadequate is debulking of polyetherimide (PEI) thermoplastic materials and the like. PEI, for example, has a typical melting temperature of 700 degrees F. and a typical softening temperature of 550 degrees F. In order for the prepreg plies to adhere to each other in a debulked state they must be debulked at about 550.degree. F. No known single automated station for debulking prepreg materials works at that temperature at a reasonable speed.
In contrast to known processes, the present invention uses relatively inexpensive tooling and is totally self contained. As compared to known systems, no large costly equipment is needed for operation. A debulking cycle, employed in accordance with the teachings of the invention, is completely controlled. Frequent clean up of tooling is not required since the tooling and other debulking apparatus are maintained at a constant temperature. Because no release agents are required, sand blasting and other follow on operations are not required.
This invention may be used on various thermoset prepreg assemblies. However, the examples of the invention described herein will focus on thermoplastic (e.g. graphite and PEI) prepreg product assemblies mainly used to produce 120 mm tank ammunition. It will be understood that the examples herein are by way of illustration and that the invention is not so limited.